Re: iSCSI : Holes in StatSN

[julian_satran@il.ibm.com]

Santosh,

Overlaps and out of order delivery and gaps are not forbidden by SAM . I
think we have to go to T10 for that I can't see a good reason to do it. We
have a good solution without asking for it .  I can see large and important
future application that will relay on overlaps and/or gaps and I am not
going to foolishly do something to disable or harm their efficient
implementation.  I think that T10s philosophy of keeping the target master
of the transfer and not limiting it in any way is too valuable to ignore.

IMHO your request violates our charter without any good reason to support
it.

Julo

Santosh Rao <santoshr@cup.hp.com> on 01/02/2001 03:53:13

Please respond to Santosh Rao <santoshr@cup.hp.com>

To:   ips@ece.cmu.edu
cc:
Subject:  Re: iSCSI : Holes in StatSN




julian_satran@il.ibm.com wrote:

> With a data sequence we may want to use a similar mechanism to ask for a
> missed data block as soon as we see one of its successors or the status.

Julian,

The missing data PDU is missing due to either a header format error, header
digest error or data digest error. [in all other cases, TCP ensures
reliable
delivery].

In 2 of the above 3 cases, [header format error & header digest error] the
initiator CANNOT do a safe interpretation of the PDU header. Without
interpreting the PDU header, the initiator does NOT get the Initiator Task
Tag. Any request to re-send a particular data PDU MUST be qualified by :
I.T.T + missing_DataSN [+ T.T.T + CmdSN, optionally].

Since I.T.T. cannot be reliably determined in 2 of the 3 cases, such a
re-send request cannot be reliably achieved.

The alternate proposal that was made should be considered in its place,
which
was to :
- dis-allow overlapped data xfer's
- initiators do a count check
- a command level retry is performed at the iSCSI layer on detecting an
underrun [due to a missing PDU].

On several ocassions, requests from different people have been made on this
list to dis-allow overlapped data xfers. Can a WG consensus be sought on
this
issue to see if the benefits of allowing overlapped data xfer's offset its
complexities and justify its support ?

Regards,
Santosh

 - santoshr.vcf